Lithium secondary battery

ABSTRACT

The present invention provides a lithium secondary battery which has improved safety, mainly coming from use of an electrolyte solution which is not inflammable at room temperature (20° C.), while not deteriorating output characteristics at low temperatures and room temperature or output maintenance characteristics after storage at high temperature (50° C.). The lithium secondary battery of the present invention, encased in a container, is provided with a cathode and an anode, both capable of storing/releasing lithium ions, a separator which separates these electrodes from each other, and an electrolyte solution containing a cyclic carbonate and a linear carbonate as solvents and a compound such as VC at composition ratios of 18.0 to 30.0%, 74.0 to 81.9% and 0.1 to 1.0%, respectively, based on the whole solvents, all percentages by volume.

BACKGROUND OF THE INVENTION

The present invention relates to a novel lithium secondary batteryhaving a high input/output performance and suitable for hybrid electricvehicles and the like.

Hybrid electric vehicles, in which an engine and a motor serve as powersources, have been developed and commercialized for environmentalprotection and energy saving. Moreover, fuel cell hybrid electricvehicles in which a fuel cell is used in place of an engine have beenextensively under development for vehicles in the future.

Secondary batteries, which can undergo a number of charging/dischargingcycles, are essential devices as power sources for hybrid electricvehicles.

Of secondary batteries, lithium secondary batteries are promising,because of their high operational voltage and capability of generating ahigh output, and are of increasing importance as power sources forhybrid electric vehicles.

An electrolyte solution for lithium secondary batteries is required tohave high voltage-resistant characteristics, and an organic electrolytesolution containing an organic solvent is used.

However, an organic solvent involves disadvantages of poor solubilityfor lithium salts and large dependence of its conductivity ontemperature, thus resulting in a marked deterioration in operationalcharacteristics at low temperatures over those at room temperature.

Carbonate ester compounds are now mainly used as solvents for organicelectrolyte solutions for lithium secondary batteries, because of theirhigh voltage-resistant characteristics.

Cyclic carbonate ester solvents have a high viscosity, although theyhave a high solubility for lithium salts. Linear carbonate estersolvents, on the other hand, have a low solubility for lithium salts,although they have a low viscosity.

Hence, a mixture of cyclic and linear carbonate esters is generally usedas the electrolyte solutions.

For improvement of low-temperature characteristics of electrolytesolutions, Patent Documents 1 to 5 shown below disclose use of a linearcarbonate ester incorporated with ethyl methyl carbonate (EMC) of anasymmetric structure.

Patent Document 6 shown below discloses use of an acetate ester, whichhas a lower molecular weight than EMC and works as a solvent of a lowviscosity and a low melting point.

Patent Document 1: JP-B-2,705,529

Patent Document 2: JP-A-10-027625

Patent Document 3: JP-A-2001-148258

Patent Document 4: JP-A-2002-305035

Patent Document 5: JP-A-2003-323915

Patent Document 6: JP-A-09-245838

It is an object of the present invention to provide a lithium secondarybattery which uses an electrolyte solution exhibiting no inflammabilityat room temperature (20° C.) and not deteriorating outputcharacteristics of the battery at low temperatures and room temperature.

SUMMARY OF THE INVENTION

The present invention provides a lithium secondary battery comprises acathode and an anode, both capable of storing/releasing a lithium ion, aseparator which separates these electrodes from each other, and anelectrolyte solution, wherein the electrolyte solution comprises assolvents:

a cyclic carbonate represented by the formula (1):

(wherein, R₁, R₂, R₃ and R₄ may be the same or different and are eachhydrogen, fluorine, chlorine, an alkyl group or a fluorinated alkylgroup, each having 1 to 3 carbons),

a linear carbonate represented by the formula (2):

(wherein, R₅ and R₆ may be the same or different and are each hydrogen,fluorine, chlorine, an alkyl group or a fluorinated alkyl group, eachhaving 1 to 3 carbons, which) and

a compound represented by the formula (3):

(wherein, R₇ and R₈ may be the same or different and are each hydrogen,fluorine, chlorine, an alkyl group or a fluorinated alkyl group, eachhaving 1 to 3 carbons), and

the composition ratio of the cyclic carbonate represented by the formula(1) is 18.0 to 30.0%, the composition ratio of the linear carbonaterepresented by the formula (2) is 74.0 to 81.9% and the compositionratio of the compound represented by the formula (3) is 0.1 to 1.0%,based on the whole solvents (100%), all percentages by volume.

The cathode mixture layer has a porosity of 25 to 40%, inclusive, andthe anode mixture layer also has a porosity of 25 to 40%, inclusive,based on the respective layers, all percentages by volume.

The cathode comprises a cathode mixture and a cathode-side currentcollector, wherein the cathode mixture comprises a cathode-activematerial, an electroconductive agent and a binder, and is spread overthe cathode-side current collector to form a cathode mixture layerthereon.

The anode comprises an anode mixture and an anode-side currentcollector, wherein the anode mixture comprises an anode-active material,an electroconductive agent and a binder, and is spread over theanode-side current collector to form an anode mixture layer thereon.

The present invention can provide a lithium secondary battery which usesan electrolyte solution exhibiting no inflammability at room temperature(20° C.) and not deteriorating output characteristics of the battery atlow temperatures and room temperature.

Other objects, features and advantages of the invention will becomeapparent from the following description of the embodiments of theinvention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a cross-sectional view illustrating one side of a spirallywound battery prepared in each of Examples; and

FIG. 2 shows changes in ion conductivity of the electrolyte solutionsused in Examples with DMC content.

DESCRIPTION OF REFERENCE NUMERALS

-   1 Cathode-side current collector-   2 Cathode mixture layer-   3 Anode-side current collector-   4 Anode mixture layer-   7 Separator-   9 Anode-side lead-   10 Cathode-side lead-   11 Cathode-side insulator-   12 Anode-side insulator-   13 Anode battery can-   14 Gasket-   15 Cathode battery lid

DETAILED DESCRIPTION OF THE INVENTION

The present invention comprises a cathode of a cathode mixturecomprising a lithium composite oxide, an electroconductive agent mainlycomposed of graphite-base carbon and a binder, spread on an aluminumfoil, wherein the cathode mixture layer has a porosity of 25 to 40% byvolume, inclusive, based on the layer. When the porosity is less than25%, the electrolyte solution may not permeate the cathode mixture layerin a sufficient quantity to decrease the number of lithium ions in thelayer. As a result, the lithium secondary battery may not produce asufficient output due to short supply of the lithium ions to thecathode-active material, in particular at low temperatures. When theporosity is more than 40%, on the other hand, the content of the cathodemixture in the layer may be insufficient to cause insufficientinput/output.

The anode for the present invention comprises an anode mixturecomprising amorphous carbon, an electroconductive agent and a binder,spread on a copper foil, wherein the anode mixture layer has a porosityof 25 to 40% by volume, inclusive, based on the layer. When the porosityis less than 25%, the electrolyte solution may not permeate the anodemixture layer in a sufficient quantity. As a result, the lithiumsecondary battery may not produce a sufficient input due to short supplyof lithium ions to the anode-active material, in particular at lowtemperatures. When the porosity is more than 40%, on the other hand, thecontent of the anode mixture in the layer may be insufficient to causeinsufficient input/output.

The solvents represented by the formula (1) include ethylene carbonate(EC), trifluoropropylene carbonate (TFPC), chloroethylene carbonate(ClEC), trifluoroethylene carbonate (TFEC), difluoroethylene carbonate(DFEC) and vinyl ethylene carbonate (VEC).

Of the above compounds, EC is more preferable, viewed from formation ofa coating film on the anode.

Incorporation of a small quantity (2% by volume or less) of ClEC, TFECor VEC imparts good cycle characteristics to a coating film on theelectrode.

Moreover, TFPC or DFEC may be incorporated in a small quantity (2% byvolume or less) for facilitating formation of a coating film on thecathode.

The solvents represented by the formula (2) include dimethyl carbonate(DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), methylpropyl carbonate (MPC), ethyl propyl carbonate (EPC), trifluoromethylethyl carbonate (TFMEC) and 1,1,1-trifluoroethyl methyl carbonate(TFEMC).

DMC is highly compatible with many solvents, and is suitable for beingmixed with EC or the like.

DEC has a lower melting point than DMC, and is suitable for improvinglow-temperature (−30° C.) characteristics.

EMC has an asymmetric structure and also a low melting point, and issuitable for improving low-temperature characteristics.

EPC and TFMEC have a propylene side chain and asymmetric structure, andare suitable as solvents for adjusting low-temperature characteristics.

TFEMC has a molecule partly fluorinated to have an increased dipolemoment, and is suitable for keeping dissociation of lithium salts at lowtemperatures and also for improving low-temperature characteristics.

The compounds represented by the formula (3) include vinylene carbonate(VC), methylvinylene carbonate (MVC), dimethylvinylene carbonate (DMVC),ethylvinylene carbonate (EVC) and diethylvinylene carbonate (DEVC).

VC has a low molecular weight, and is considered to form a dense coatingfilm on the electrode. MVC, DMVC, EVC, DEVC or the like is a VCsubstituted with an alkyl group and is considered to form a low-densitycoating film on the electrode, magnitude of density depending on size ofthe alkyl chain with which the compound is substituted, thus to have theeffect of improving low-temperature characteristics.

The lithium salt for the electrolyte solution is not limited. Thelithium salts useful for the present invention include inorganic lithiumsalts, e.g., LiPF₆, LiBF₄, LiClO₄, LiI, LiCl and LiBr; and organiclithium salts, e.g., LiB[OCOCF₃]₄, LiB[OCOCF₂CF₃]₄, LiPF₄(CF₃)₂,LiN(SO₂CF₃)₂ and LiN(SO₂CF₂CF₃)₂.

In particular, LiPF₆, which has been widely used for batteries fordomestic purposes, is a suitable compound because of its qualitystability.

Moreover, LiB[OCOCF₃]₄ is an effective compound, because it exhibitshigh dissociation capability and solubility, and also high conductivityeven at a low content.

An electrolyte solution preferably has a flash point of 21° C. or higherand an ion conductivity of 2 mS/cm or more at −30° C. to simultaneouslysatisfy safety and low-temperature characteristics.

It is preferable to incorporate DMC and EMC in a ratio of 0.6 to 1.3 byvolume, particularly preferably 0.8 to 1. A DMC/EMC ratio of 0.6 to 1.3will secure an electrolyte solution having a flash point of 21° C. orhigher and an ion conductivity of 2 mS/cm or more at −30° C. Moreover, aDMC/EMC ratio of 0.8 to 1 will secure an electrolyte solution having aflash point of 21° C. or higher and an ion conductivity of 2.2 mS/cm ormore at −30° C., and hence is more preferable.

The anode-active materials useful for the present invention includenatural graphite, composite carbonaceous material with natural graphitecoated with a film formed by dry chemical vapor deposition (CVD) or wetspraying, synthetic graphite produced by sintering a resin (e.g., epoxyor phenolic resin) or pitch from petroleum or coal, another carbonaceousmaterial (e.g., amorphous material), lithium metal having a capabilityof storing/releasing lithium by forming a compound with lithium, siliconreacting with lithium to form a compound to have a capability ofstoring/releasing lithium by being held in the interstices between thecrystals, and oxide or nitride of an element belonging to the IV group(e.g., germanium or tin). They may be generally referred to asanode-active materials. In particular, a carbonaceous material is anexcellent material because of its high electroconductivity,low-temperature characteristics and good cycle stability.

Of carbonaceous materials, those having a wide interlayer space betweencarbon network planes (d₀₀₂) are suitable for their rapidcharging/discharging capability and excellent low-temperaturecharacteristics.

It should be noted, however, that some carbonaceous materials having awide d₀₀₂ value show insufficient capacity or charging/dischargingefficiency during the initial stage of charging, and hence theypreferably have a d₀₀₂ value of 0.39 nm or less. Such a material may besometimes referred to as pseudo-anisotropic carbon.

Moreover, the electrode may be incorporated with a highlyelectroconductive carbonaceous material, e.g., graphite-like material,amorphous material or activated carbon.

Graphite-like materials useful for the present invention include thosehaving one of the following characteristics (1) to (3):

(1) an R value, or I_(D)/I_(G) ratio, of 0.2 to 0.4, inclusive, whereinI_(D) is intensity of the peak in a range from 1300 to 1400 cm⁻¹, andI_(G) is intensity of the peak in a range from 1580 to 1620 cm⁻¹, bothin a Raman spectral pattern,

(2) a half width Δ of 40 to 100 cm⁻¹, inclusive, of the peak in a rangefrom 1300 to 1400 cm⁻¹ in a Raman spectral pattern, and

(3) an X value, or I(₁₁₀)/I(₀₀₄) ratio, of 0.1 to 0.45, inclusive,wherein I(₁₁₀) is peak intensity from the (110) plane and I(₀₀₄) is peakintensity from the (004) plane, both in an X-ray diffraction pattern.

A lithium composite oxide as the cathode-active material preferably hasa composition represented by the formula Li_(α)Mn_(x)M1_(y)M2_(z)O₂(wherein, M1 is at least one selected from the group consisting of Coand Ni, M2 is at least one selected from the group consisting of Co, Ni,Al, B, Fe, Mg and Cr, x+y+z=1, 0<α<1.2, 0.2≦x≦0.6, 0.2≦y≦0.4 and0.05≦z≦0.4).

Of these materials, those having Ni or Co as M1, and Co or Ni as M2 aremore preferable. LiMn_(1/3)Ni_(1/3)Co_(1/3)O₂ is still more preferable.The composition has an increased capacity as the Ni content increases,an increased output at low temperatures as the Co content increases, anddecreases in production cost as the Mn content increases. The additiveelements have effects of improving cycle characteristics. Othercompounds, e.g., those represented by the general formula LiM_(x)PO₄ (M:Fe or Mn, and 0.01≦X≦0.4), and orthorhombic phosphorus compounds havingspace group Pmnb symmetry represented by LiMn_(1-x)M_(x)PO₄ (M: divalentcation other than Mn, and 0.01≦X≦0.4) may be also used for the presentinvention.

LiMn_(1/3)Ni_(1/3)Co_(1/3)O₂, in particular, is suitable as a lithiumbattery material for hybrid electric vehicles (HEVs) because of its highlow-temperature characteristics and cycle stability.

The lithium secondary battery described above as one embodiment of thepresent invention has improved safety over conventional lithiumsecondary batteries, mainly coming from use of an electrolyte solutionwhich is not inflammable at room temperature (20° C.) while notdeteriorating output characteristics at low temperatures and roomtemperature or output maintenance characteristics during storage at hightemperatures over an extended period. As such, it can find wide use invarious areas, e.g., power sources for hybrid electric vehicles, andpower sources including back-up power sources for electrically drivencontrol systems for vehicles. Moreover, it is also suitable as a powersource for industrial machines, e.g., electrically driven tools andforklifts.

Moreover, the lithium secondary battery as one embodiment of the presentinvention has improved characteristics, in particular outputcharacteristics at low temperatures, and useful for vehicles frequentlyworking in cold districts. The improved output characteristics at lowtemperatures reduce size and weight of a module in which the batteriesare assembled to produce power of several hundred volts by reducing arequired number of the batteries.

The best mode for carrying out the present invention is specificallydescribed by Examples.

Example 1

An electrolyte solution was prepared by dissolving LiPF₆ as a lithiumsalt to a concentration of 1 M in a mixed solvent of EC:VC:DMC:EMChaving a composition ratio of 19.4:0.6:40:40 by volume. The electrolytesolution was not inflammable as confirmed by the flash point testcarried out at 20° C.

Flash point was determined using an automatic tag closed flash pointtester in accordance with JIS K-2265.

(Evaluation of Ion Conductivity)

The sample solution (5 mL) put in a glass bottle was measured for itsion conductivity at −30° C. by a digital electroconductivity meter(CM-60V, Toa Denpa) whose electrodes were immersed in the samplesolution, where the measurement was started 90 minutes after the bottlewas held in a constant-temperature bath.

The electrolyte solution preferably has a flash point of 21° C. orhigher and an ion conductivity of 2 mS/cm or more at −30° C. tosimultaneously satisfy safety and low-temperature characteristics. FIG.2 shows an ion conductivity of mixed electrolyte solutions ofEC/VC/DMC/EMC having a composition ratio of 19.4/0.6/X/80-X % by volume,changing with the DMC/EMC volume ratio. The results indicate that theelectrolyte solution has a flash point of 21° C. or higher and an ionconductivity of 2 mS/cm or more at −30° C. when the DMC/EMC volume ratiois set at 0.6 to 1.3. The ratio of 0.8 to 1 is more preferable, becauseit gives a flash point of 21° C. or higher and an ion conductivity of2.2 mS/cm or more at −30° C.

(Fabrication of Spirally Wound Battery)

A spirally wound battery of Example 1 was fabricated by the followingprocedure. FIG. 1 is a cross-sectional view illustrating one side of thebattery.

A cathode material paste was prepared using LiMn_(1/3)Ni_(1/3)Co_(1/3)O₂as a cathode-active material, carbon black (CB1) and graphite (GF1) aselectroconductive materials, polyvinylidene fluoride (PVDF) as a binder,and N-methylpyrrolidone (NMP) as a solvent to have a composition ratioof LiMn_(1/3)Ni_(1/3)Co_(1/3)O₂:CB1:GF1:PVDF of 86:9:2:3 by mass (drybasis).

The cathode material paste was spread over an aluminum foil, whichbecame a cathode-side current collector 1, dried at 80° C.,roll-pressed, and dried at 120° C. to prepare a cathode mixture layer 2on the cathode-side current collector 1. The cathode mixture layer had aporosity set at 30% based on the whole layer (the value is hereinafterreferred to as cathode porosity).

Next, an anode material paste was prepared using pseudo-anisotropiccarbon as amorphous carbon serving as an anode-active material, carbonblack (CB2) as an electroconductive material, PVDF as a binder and NMPas a solvent, to have a composition of pseudo-anisotropiccarbon:CB2:PVDF of 88:5:7 by mass (dry basis).

The anode material paste was spread over a copper foil, which became ananode-side current collector 3, dried at 80° C., roll-pressed, and driedat 120° C. to prepare an anode mixture layer 4 on the anode-side currentcollector 3. The anode mixture layer had a porosity set at 35% based onthe whole layer (the value is hereinafter referred to as anodeporosity), so as to form a battery.

A separator 7 was placed between the cathode and anode prepared above toform a spirally wound assembly, which was encased in an anode batterycan 13. The electrolyte solution prepared in Example 1 was injected intothe assembly and sealed by caulking to form a spirally wound battery.

The other components shown in FIG. 1 are 9: anode-side lead, 10:cathode-side lead, 11: cathode-side insulator, 12: anode-side insulator,14: gasket, and 15: cathode battery lid.

(Evaluation of Porosity)

Porosity was determined by mercury porosimetry for the cathode and anodeafter the mixture layer was separated from the current collector of eachside.

(Evaluation of Battery)

The spirally wound battery illustrated in FIG. 1 was evaluated fordirect current resistance (DCR) at 25 and −30° C., and pulse cyclecharacteristics (characteristics after the battery was subjected topulse cycles for 1000 hours).

The battery was charged with electricity at a constant current of 0.7 Ato 4.1 V, and then at a constant voltage of 4.1 V until the amperagereached 20 mA. Then, it was allowed to discharge electricity to 2.7 V at0.7 A, after it was halted for 30 minutes. These cycles were repeated 3times.

Next, the battery was charged with electricity at a constant current of0.7 A to 3.8 V, allowed to discharge electricity at 10 A for 10 seconds,again charged with electricity to 3.8 V at a constant current, allowedto discharge electricity at 20 A for 10 seconds, again charged withelectricity to 3.8 V, and allowed to discharge electricity at 30 A for10 seconds.

The battery was evaluated for DCR based on the I-V characteristicsobserved.

Moreover, the battery was subjected to a pulse cycle test in whichcharging/discharging were repeated at 20 A for 2 seconds in aconstant-temperature bath kept at 50° C., and evaluated for DCR at 25and −30° C., after it was subjected to the pulse cycles for 1000 hours.The evaluation results are given in Table 1.

Example 2

An electrolyte solution was prepared by dissolving LiPF₆ as a lithiumsalt to a concentration of 1 M in a mixed solvent of EC:VC:DMC:EMChaving a composition ratio of 19.5:0.5:40:40 by volume, and evaluatedfor ion conductivity and inflammability in the same manner as in Example1.

A spirally wound battery was prepared, and evaluated for porosity andbattery characteristics in the same manner as in Example 1. It wasconfirmed to have a cathode porosity of 30% and anode porosity of 35%,all percentages by volume. The evaluation results are given in Table 1.

Example 3

An electrolyte solution was prepared by dissolving LiPF₆ as a lithiumsalt to a concentration of 1 M in a mixed solvent of EC:VC:DMC:EMChaving a composition ratio of 19.6:0.4:40:40 by volume, and evaluatedfor ion conductivity and inflammability in the same manner as in Example1.

A spirally wound battery was prepared, and evaluated for porosity andbattery characteristics in the same manner as in Example 1. It wasconfirmed to have a cathode porosity of 30% and anode porosity of 35%,all percentages by volume. The evaluation results are given in Table 1.

Example 4

An electrolyte solution was prepared by dissolving LiPF₆ as a lithiumsalt to a concentration of 1 M in a mixed solvent of EC:VC:DMC:EMChaving a composition ratio of 19.8:0.2:40:40 by volume, and evaluatedfor ion conductivity and inflammability in the same manner as in Example1.

A spirally wound battery was prepared, and evaluated for porosity andbattery characteristics in the same manner as in Example 1. It wasconfirmed to have a cathode porosity of 30% and anode porosity of 35%,all percentages by volume. The evaluation results are given in Table 1.

Example 5

An electrolyte solution was prepared by dissolving LiPF₆ as a lithiumsalt to a concentration of 1 M in a mixed solvent of EC:VC:DMC:EMChaving a composition ratio of 19.7:0.3:40:40 by volume, and evaluatedfor ion conductivity and inflammability in the same manner as in Example1.

A spirally wound battery was prepared, and evaluated for porosity andbattery characteristics in the same manner as in Example 1. It wasconfirmed to have a cathode porosity of 30% and anode porosity of 35%,all percentages by volume. The evaluation results are given in Table 1.

Example 6

An electrolyte solution was prepared by dissolving LiPF₆ as a lithiumsalt to a concentration of 1 M in a mixed solvent of EC:VC:DMC:EMChaving a composition ratio of 21.7:0.3:38:40 by volume, and evaluatedfor ion conductivity and inflammability in the same manner as in Example1.

A spirally wound battery was prepared, and evaluated for porosity andbattery characteristics in the same manner as in Example 1. It wasconfirmed to have a cathode porosity of 30% and anode porosity of 35%,all percentages by volume. The evaluation results are given in Table 1.

Example 7

An electrolyte solution was prepared by dissolving LiPF₆ as a lithiumsalt to a concentration of 1 M in a mixed solvent of EC:VC:DMC:EMChaving a composition ratio of 23.7:0.3:36:40 by volume, and evaluatedfor ion conductivity and inflammability in the same manner as in Example1.

A spirally wound battery was prepared, and evaluated for porosity andbattery characteristics in the same manner as in Example 1. It wasconfirmed to have a cathode porosity of 30% and anode porosity of 35%,all percentages by volume. The evaluation results are given in Table 1.

Example 8

An electrolyte solution was prepared by dissolving LiPF₆ as a lithiumsalt to a concentration of 1 M in a mixed solvent of EC:VC:DMC:EMChaving a composition ratio of 29.7:0.3:30:40 by volume, and evaluatedfor ion conductivity and inflammability in the same manner as in Example1.

A spirally wound battery was prepared, and evaluated for porosity andbattery characteristics in the same manner as in Example 1. It wasconfirmed to have a cathode porosity of 30% and anode porosity of 35%,all percentages by volume. The evaluation results are given in Table 1.

Example 9

An electrolyte solution was prepared by dissolving LiPF₆ as a lithiumsalt to a concentration of 1.2 M in a mixed solvent of EC:VC:DMC:EMChaving a composition ratio of 23.7:0.3:36:40 by volume, and evaluatedfor ion conductivity and inflammability in the same manner as in Example1.

A spirally wound battery was prepared, and evaluated for porosity andbattery characteristics in the same manner as in Example 1. It wasconfirmed to have a cathode porosity of 30% and anode porosity of 35%,all percentages by volume. The evaluation results are given in Table 1.

Example 10

An electrolyte solution was prepared by dissolving LiPF₆ as a lithiumsalt to a concentration of 1.1 M in a mixed solvent of EC:VC:DMC:EMChaving a composition ratio of 23.7:0.3:36:40 by volume, and evaluatedfor ion conductivity and inflammability in the same manner as in Example1.

A spirally wound battery was prepared, and evaluated for porosity andbattery characteristics in the same manner as in Example 1. It wasconfirmed to have a cathode porosity of 30% and anode porosity of 35%,all percentages by volume. The evaluation results are given in Table 1.

Example 11

An electrolyte solution was prepared by dissolving LiPF₆ as a lithiumsalt to a concentration of 1.1 M in a mixed solvent of EC:VC:DMC:EMChaving a composition ratio of 23.7:0.3:36:40 by volume, and evaluatedfor ion conductivity and inflammability in the same manner as in Example1.

A spirally wound battery was prepared, and evaluated for porosity andbattery characteristics in the same manner as in Example 1. It wasconfirmed to have a cathode porosity of 25% and anode porosity of 25%,all percentages by volume. The evaluation results are given in Table 1.

Example 12

An electrolyte solution was prepared by dissolving LiPF₆ as a lithiumsalt to a concentration of 1.1 M in a mixed solvent of EC:VC:DMC:EMChaving a composition ratio of 23.7:0.3:36:40 by volume, and evaluatedfor ion conductivity and inflammability in the same manner as in Example1.

A spirally wound battery was prepared, and evaluated for porosity andbattery characteristics in the same manner as in Example 1. It wasconfirmed to have a cathode porosity of 35% and anode porosity of 40%,all percentages by volume. The evaluation results are given in Table 1.

Example 13

An electrolyte solution was prepared by dissolving LiPF₆ as a lithiumsalt to a concentration of 1.1 M in a mixed solvent of EC:VC:DMC:EMChaving a composition ratio of 23.7:0.3:36:40 by volume, and evaluatedfor ion conductivity and inflammability in the same manner as in Example1.

A spirally wound battery was prepared, and evaluated for porosity andbattery characteristics in the same manner as in Example 1. It wasconfirmed to have a cathode porosity of 40% and anode porosity of 40%,all percentages by volume. The evaluation results are given in Table 1.

Comparative Example 1

A comparative electrolyte solution was prepared by dissolving LiPF₆ as alithium salt to a concentration of 1 M in a mixed solvent ofEC:VC:DMC:EMC:methyl acetate (MA) having a composition ratio of29.4:0.6:30:30:10 by volume, and evaluated for ion conductivity andinflammability in the same manner as in Example 1.

A spirally wound battery was prepared, and evaluated for porosity andbattery characteristics in the same manner as in Example 1. It wasconfirmed to have a cathode porosity of 30% and anode porosity of 35%,all percentages by volume. The evaluation results are given in Table 1.

Comparative Example 2

A comparative electrolyte solution was prepared by dissolving LiPF₆ as alithium salt to a concentration of 1 M in a mixed solvent ofEC:VC:DMC:EMC having a composition ratio of 34:0:33:33 by volume, andevaluated for ion conductivity and inflammability in the same manner asin Example 1.

A spirally wound battery was prepared, and evaluated for porosity andbattery characteristics in the same manner as in Example 1. It wasconfirmed to have a cathode porosity of 30% and anode porosity of 35%,all percentages by volume. The evaluation results are given in Table 1.

Comparative Example 3

A comparative electrolyte solution was prepared by dissolving LiPF₆ as alithium salt to a concentration of 1 M in a mixed solvent ofEC:VC:DMC:EMC having a composition ratio of 15:0:35:50 by volume, andevaluated for ion conductivity and inflammability in the same manner asin Example 1.

A spirally wound battery was prepared, and evaluated for porosity andbattery characteristics in the same manner as in Example 1. It wasconfirmed to have a cathode porosity of 30% and anode porosity of 35%,all percentages by volume. The evaluation results are given in Table 1.

Comparative Example 4

A comparative electrolyte solution was prepared by dissolving LiPF₆ as alithium salt to a concentration of 1 M in a mixed solvent ofEC:VC:DMC:EMC having a composition ratio of 18:2:40:40 by volume, andevaluated for ion conductivity and inflammability in the same manner asin Example 1.

A spirally wound battery was prepared, and evaluated for porosity andbattery characteristics in the same manner as in Example 1. It wasconfirmed to have a cathode porosity of 30% and anode porosity of 35%,all percentages by volume. The evaluation results are given in Table 1.

Comparative Example 5

A comparative electrolyte solution was prepared by dissolving LiPF₆ as alithium salt to a concentration of 1.1 M in a mixed solvent ofEC:VC:DMC:EMC having a composition ratio of 23.7:0.3:36:40 by volume,and evaluated for ion conductivity and inflammability in the same manneras in Example 1.

A spirally wound battery was prepared, and evaluated for porosity andbattery characteristics in the same manner as in Example 1. It wasconfirmed to have a cathode porosity of 20% and anode porosity of 20%,all percentages by volume. The evaluation results are given in Table 1.

Comparative Example 6

A comparative electrolyte solution was prepared by dissolving LiPF₆ as alithium salt to a concentration of 1.1 M in a mixed solvent ofEC:VC:DMC:EMC having a composition ratio of 23.7:0.3:36:40 by volume,and evaluated for ion conductivity and inflammability in the same manneras in Example 1.

A spirally wound battery was prepared, and evaluated for porosity andbattery characteristics in the same manner as in Example 1. It wasconfirmed to have a cathode porosity of 38% and anode porosity of 40%,all percentages by volume. The evaluation results are given in Table 1.

TABLE 1 Pulse cycle characteristics after subjected to pulse cycles for1000 hours Ion Initial Output Output Electrolyte solution compositionPorosity conduc- characteristics maintenance maintenance EC VC DMC EMCMA (vol %) Inflam- tivity DCR DCR ratio ratio LiPF₆ (vol (vol (vol (vol(vol Cath- An- mability @−30° C. @25° C. @−30° C. @25° C. @−30° C. (M)%) %) %) %) %) ode ode at 20° C. (mS/cm) (mΩ) (mΩ) (%) (%) Example 1 119.4 0.6 40 40 0 30 35 Not observed 2.3 67 610 84 83 Example 2 1 19.50.5 40 40 0 30 35 Not observed 2.3 66 603 85 84 Example 3 1 19.6 0.4 4040 0 30 35 Not observed 2.3 66 595 85 83 Example 4 1 19.8 0.2 40 40 0 3035 Not observed 2.3 65 595 86 85 Example 5 1 19.7 0.3 40 40 0 30 35 Notobserved 2.3 63 590 88 87 Example 6 1 21.7 0.3 38 40 0 30 35 Notobserved 2.3 63 590 88 87 Example 7 1 23.7 0.3 36 40 0 30 35 Notobserved 2.3 62 592 88 87 Example 8 1 29.7 0.3 30 40 0 30 35 Notobserved 2.0 62 615 84 83 Example 9 1.2 23.7 0.3 36 40 0 30 35 Notobserved 2.2 63 592 88 87 Example 10 1.1 23.7 0.3 36 40 0 30 35 Notobserved 2.2 61 592 88 87 Example 11 1.1 23.7 0.3 36 40 0 25 25 Notobserved 2.2 63 600 88 87 Example 12 1.1 23.7 0.3 36 40 0 35 40 Notobserved 2.2 65 610 88 87 Example 13 1.1 23.7 0.3 36 40 0 40 40 Notobserved 2.2 66 620 88 87 Comparative 1 29.4 0.6 30 30 10 30 35 Observed2.3 68 675 83 82 Example 1 Comparative 1 34 0 33 33 0 30 35 Not observed1.9 68 850 72 69 Example 2 Comparative 1 15 0 35 50 0 30 35 Not observed2.3 75 680 68 67 Example 3 Comparative 1 18 2 40 40 0 30 35 Not observed2.3 70 750 83 82 Example 4 Comparative 1.1 23.7 0.3 36 40 0 20 20 Notobserved 2.2 70 730 82 75 Example 5 Comparative 1.1 23.7 0.3 36 40 0 4545 Not observed 2.2 70 700 83 77 Example 6

The battery prepared in Comparative Example 1 with an electrolytesolution containing MA exhibits good characteristics, in particularlower DCR at −30° C. than the other Comparative Examples; with MA-freeelectrolyte solutions. Nevertheless, it is not preferable because ofinflammability observed at 20° C., and lower pulse cycle characteristicsafter it is subjected to pulse cycles for 1000 hours than those preparedin Examples 1 to 13.

The battery prepared in Comparative Example 2 with an electrolytesolution containing EC at a higher content of 34% by volume than thoseprepared in Examples 1 to 13 is not preferable, because of its higherDCR at −30° C.

The battery prepared in Comparative Example 3 with an electrolytesolution containing EC at a lower content of 15% by volume than thoseprepared in Examples 1 to 13 is not preferable, because of its lower DCRat 25° C.

The battery prepared in Comparative Example 4 with an electrolytesolution containing VC at a higher content of 2% by volume than thoseprepared in Examples 1 to 13 is not preferable, because of its lower DCRat 25 and −30° C.

The battery prepared in Comparative Example 5 with lower porosities of20% by volume in the cathode and anode than those prepared in Examples 1to 13 is not preferable, because of its higher DCR at 25 and −30° C.

The battery prepared in Comparative Example 6 with a higher porosity of45% by volume in each of the cathode and anode than those prepared inExamples 1 to 13 is not preferable, because of its higher DCR at 25 and−30° C.

As discussed above, each of Examples 1 to 13 can provide a lithiumsecondary battery which exhibits improved safety, in particular with anelectrolyte solution exhibiting no inflammability at room temperature(20° C.), while not deteriorating output characteristics at lowtemperatures and room temperature, and output maintenancecharacteristics when the battery is preserved at high temperature (50°C.).

Moreover, the lithium secondary battery prepared in each of Examples 1to 13 as a high-output battery has improved DCR at low temperatures andlow-temperature input/output characteristics over conventional ones.

Still more, the lithium secondary battery prepared in each of Examples 1to 13 has improved characteristics, in particular output characteristicsat low temperatures, which make the battery effective for use invehicles frequently working in cold districts. The improved outputcharacteristics at low temperatures bring advantages of reducing sizeand weight of a module in which the batteries are assembled to producepower of several hundred volts by reducing a required number of thebatteries.

It should be further understood by those skilled in the art thatalthough the foregoing description has been made on embodiments of theinvention, the invention is not limited thereto and various changes andmodifications may be made without departing from the spirit of theinvention and the scope of the appended claims.

The invention claimed is:
 1. A lithium secondary battery provided with acathode and an anode, both capable of storing/releasing lithium ions, aseparator which separates these electrodes from each other, and anelectrolyte solution, wherein solvents of the electrolyte solutionconsist of ethylene carbonate (EC) in an amount of 18.0 to 30.0% byvolume, dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC) in anamount of 74.0 to 81.9% by volume, wherein a DMC/EMC ratio by volume is0.6 to 1.3, and vinylene carbonate (VC) in an amount of 0.1 to 1.0% byvolume based on the whole solvents.
 2. The lithium secondary batteryaccording to claim 1, wherein the cathode comprises a lithium compositeoxide represented by the formula LiMn_(x)M1_(y)M2_(z)O₂ (wherein, M1 isat least one selected from the group consisting of Co and Ni, M2 is atleast one selected from the group consisting of Co, Ni, Al, B, Fe, Mgand Cr, x+y+z=1, 0.2≦x≦0.6, 0.2≦y≦4 and 0.05≦z≦4).
 3. The lithiumsecondary battery according to claim 1, wherein the anode comprises atleast one of carbonaceous material, an oxide of an element belonging tothe IV group and a nitride of an element belonging to the IV group. 4.The lithium secondary battery according to claim 1, wherein the cathodecomprises a cathode mixture and a cathode-side current collector, thecathode mixture comprising a cathode-active material, anelectroconductive agent and a binder is spread on the cathode-sidecurrent collector to form a cathode mixture layer thereon, the cathodemixture layer having a porosity of 25 to 40% by volume, inclusive, basedon the layer, the anode comprises an anode mixture and an anode-sidecurrent collector, and the anode mixture comprising an anode-activematerial, an electroconductive agent and a binder is spread on theanode-side current collector to form an anode mixture layer thereon, theanode mixture layer having a porosity of 25 to 40% by volume, inclusive,based on the layer.
 5. The lithium secondary battery according to claim1, wherein the DMC and EMC are incorporated in a DMC/EMC ratio of 0.8 to1.0 by volume.
 6. The lithium secondary battery according to claim 1,wherein the anode comprises a carbonaceous material having a d₀₀₂ valueof 0.39 nm or less.
 7. The lithium secondary battery according to claim5, wherein the cathode is formed by spreading, on an aluminum foil, acathode mixture comprising a lithium composite oxide, anelectroconductive agent mainly composed of graphite-base carbonaceousmaterial and a binder, the cathode mixture layer has a porosity of 25 to40% by volume, inclusive, based on the layer, the anode is formed byspreading, on a copper foil, an anode mixture comprising amorphouscarbon, an electroconductive agent and a binder, and the anode mixturelayer has a porosity of 25 to 40% by volume, inclusive, based on thelayer.
 8. The lithium secondary battery according to claim 7, whereinthe electrolyte solution has a flash point of 20° C. or higher and anion conductivity of 2 mS/cm or more at −30° C.